Generally, high-output, high-heat generation semiconductor devices such as power MOS (Metal Oxide Semiconductor) transistors, IGBT (Insulated-Gate Bipolar Transistors) or other power transistors as well as power ICs (Integrated Circuits) are used in every field of electronic and electrical equipment including power supplies and switches of battery-driven equipment, automobile-use electrical equipment, motor drive-use control units, and the like.
One example of conventional such high-output, high-heat generation semiconductor devices is disclosed in JP 59-25256 A (Patent Literature 1). This conventional semiconductor device, as shown in FIG. 5, includes a lead frame 51 having a header portion 54 and three lead portions 55.
The header portion 54 has a power transistor 56 mounted thereon and releases heat of the power transistor 56. The header portion 54 is provided as one unit integrated with one lead portion 55.
Electrode pads of the power transistor 56 are electrically connected to the other two lead portions 55 by wires 57. Then, the power transistor 56 is sealed together with the header portion 54 by a resin sealant 52.
Another conventional semiconductor device is disclosed in JP 3685659 B (Patent Literature 2). In this semiconductor device, as shown in FIG. 6, a protrusive electrode 62 is formed on a power transistor 63, and a lead portion 66 of a lead frame 69 is connected to the electrode 62 with solder. It is noted that numeral 68 in FIG. 6 denotes a resin sealant.
For the conventional semiconductor device of Patent Literature 1 above, the wires 57 and their junction members (electrode pads of the power transistor 56 and lead portions 55) are deformed by application of ultrasonic waves, and those members are connected to each other under the condition that oxide films present on their surfaces have been removed. However, there are fears for such adverse effects that the application of ultrasonic waves may cause an insulation film under the electrode pad to be broken or the electrode pad to be peeled off. Thus, the formation of their optimum connections necessitates condition optimization and enough process control.
Particularly, use of the wire 57 involves force concentration at an end portion of the wire 57, causing a fear for occurrence of insulation film breakdown or the like and leading to increased difficulty in setting wire-bonding conditions. Although a method using ribbon instead of the wire 57 is also available, which allows stress concentration to be reduced, yet giving ultrasonic waves to the whole ribbon may cause occurrence of a wide range of peeling. As another method for connection other than that using ultrasonic waves, the wire 57 and the electrode pad may be connected together by application of heat and pressure such as in thermo-compression bonding. In this case also, there is a fear for adverse effects such as breakdown of the dielectric layer under the electrode pad or peeling of the electrode pad or the like due to the pressure and heat.
Particularly with a structure that the insulation film on the surface of the power transistor 56 is formed of a resin which is low in adhesion with metal such as polyimide resin and which has a Young's modulus of 100 GPa or less, with the electrode pad formed on the insulation film, there arises a problem that attenuation of ultrasonic waves leads to insufficient friction between the electrode pad and the wire 57 so that the metal pad and the wire 57 are not bonded together, or a fear for a problem that increasing the power for the ultrasonic waves causes the electrode pad to be peeled from the insulation film, thus making it quite difficult to meet the required conditions. Also, in a case where the insulation film on the surface of the power transistor 56 is formed from polyimide resin while a nitride film is formed on the polyimide resin, with an improved adhesion between the polyimide resin and the electrode pad, there is a fear that the nitride film may be peeled from polyimide resin as a result of cracks that may occur to the nitride film due to deformation of the polyimide resin, which the nitride film cannot follow because the nitride film has as high a hardness as a Young's modulus of 270 GPa while polyimide resin has a Young's modulus of 27 GPa. This causes the importance of process control to be increased.
In the conventional semiconductor device of Patent Literature 2, since an electrode of thick film is formed for the formation of the protrusive electrode 62 by using semiconductor process, there is a fear for increases in manufacturing cost for the preparation of larger than 100 μm protrusions. While a method of attaching a solder ball to the surface of the power transistor 63 is also available, there is a fear for a problem of peeling of the electrode similar to that in the conventional semiconductor device of Patent Literature 1.
Moreover, connecting all the terminals on the surface of the power transistor 63 to the lead portions 66 of the lead frame 69 necessitates clearances between the lead portions 66 in order to avoid contact among the lead portions 66. This gives rise to a fear for increases in the chip size of the power transistor 63.